Passive Solar Heating Basics

Categories: Construction Methods


A direct gain passive solar heating system needs to have plenty of south facing glass and adequate thermal storage capacity in the living space. A guideline sometimes used for thermal storage capacity is to construct one-half to two-thirds of the total interior surface area of thermal storage materials. Materials typically used for thermal storage are masonry, such as concrete, adobe, brick, etc. Also water walls can be used as well. A water wall consists of water in plastic or metal containers placed in the direct path of the sunlight. Water walls heat more quickly and more evenly than masonry, but they may not be as pleasing aesthetically. The surface temperature of dark colored masonry surfaces may become quite high if they receive direct sunlight. One way to avoid this, is to use a diffusing glazing material which scatters sunlight, thus distributing it more evenly over walls, ceiling, and floor. This approach distributes the incoming solar radiation more evenly but doesn't reduce the total amount of solar energy entering the space. The effect of using a diffusing glazing material is illustrated in Figure 2, below.Nighttime heat loss can be reduced by the use ofmoveable insulation, such as insulated drapes, to cover the south facing windows at night. 

A Thermal Storage Wall is shown in Figure 3 below. These walls are typically made of masonry or of water filled containers. This type of wall is sometimes called a Trombe Wall. It is named after the engineer, Felix Trombe, who popularized the design together with architect, Jacques Michel, in the 1960’s. In 1881, this design was patented by Edward Morse. The key feature of the thermal storage (Trombe) wall system is the presence of thermal storage material between the interior living space and the sun. As a result of this wall placement, the dark colored storage wall will be heated by sunshine during day and the stored heat in the wall will provide heat to the living space at night. Vents are typically placed near the top and the bottom of the wall, as shown in Figure 3. These vents should be opened during the day and closed at night, to provide natural convection* heating of the living space from the heated air between the storage wall and glazing during sunlight hours and to minimize heat loss from the heated space at night. As with the direct gain system, use of movable insulation for the glazing at night will reduce nighttime heat loss. *NOTE: Natural convection (also called free convection) is the movement of a fluid because of the reduced density of a heated portion of the fluid. That is, as a portion of a fluid is heated, its density decreases and it rises. This will cause movement of other portions of a fluid mass as well. In the thermal storage wall, for example, the heated air between the glazing and the storage wall will rise and enter the living space through the upper vent. This will draw cool air from the bottom of the living area into the space between the glazing and storage wall through the lower vent. 

An Attached Sunspace passive solar heating system. A useful feature of this type of system is that it can be added to an existing building. This makes it more suitable for retrofit than some of the other types of passive heating systems. This type of system uses direct gain in the sunspace, which is sometimes called a solar greenhouse, and may, in fact, be used as a greenhouse. Another important part of the attached sunspace system is a thermal storage wall between the sunspace and the rest of the living space, as shown in Figure 4. Vents are typically included at the top and bottom of the thermal storage wall, as described above for a thermal storage (Trombe) wall. 

The Thermal Storage Roof, also called a solar roof, solar pond, or roof pond, uses water encased in plastic on the roof. Moveable insulation is used to cover the roof and reduce heat loss during the night, but the roof must be uncovered during the daytime to allow the sunshine to strike the pond. The pond (water encased in plastic) can also be placed in an attic, under glazing in a pitched roof. Figure 5 shows typical daytime and nighttime heat flows for a thermal storage roof system. On the minus side, however, a thermal storage roof requires a somewhat elaborate drainage system, movable insulation to cover and uncover the water at appropriate times, and a structural system to support up to 65 lbs/sq ft dead load. 

A convective loop system uses a flat plate solar collector to heat air or water. The heater air or water then flows by natural convection to directly heat a living space or to a thermal storage area. Two convective loop configurations for space heating are shown in Figure 6. One of the systems has a vertically mounted collector, and the other has the collector mounted at a tilt. Either of these systems can be mounted on an existing wall, so they are quite suitable for retrofit applications. Either of these systems needs two openings into the building, one at the top of the collector for heated air flow into the building, and one at the bottom of the collector for cool air flow from the building into the collector. Some applications use a window on one floor for the upper opening and a window on a lower floor for the bottom opening. The vertically mounted collector, as shown at the right in Figure 6, is also referred to as a solar chimney. Other names used for any of the convective loop systems are isolated gain and thermosiphon system. 

Inputs Needed to Estimate Size/Performance of a Passive Solar Heating System In planning and designing a passive solar heating system, it is often helpful to estimate the performance of a particular passive solar heating system, or to estimate the size system needed to provide a specified percentage of the heating requirements for a building. In order to do either of these, information is typically needed on each of the following: i) heating requirements (degree days) during the heating season at the site of interest, ii) information on the rate of heat loss from the house, and iii) available solar radiation at the site of interest. Sources of information for, or means of estimating those three items, will be discussed in this section. Then the use of that information for sizing and performance calculations will be covered in the next section. i) Heating requirements (degree days) during the heating season at the site of interest: Data on heating degree days is available from a variety of sources.

Two such sources will be discussed here. One very good source of data for passive solar heating applications is Solar Radiation Data Manual for Buildings, published by the National Renewable Energy Laboratory (NREL), and available for free download at the website: This publication includes data for 239 locations in the United States and its territories, based on data collected from 1961-1990 for those 239 sites. Monthly average and yearly average values for heating degree days are given in the Solar Radiation Data Manual for Buildings for each of the 239 sites covered in that publication. The other source for passive solar heating data that will be discussed in this section, is the second reference in the “Related Links and References” for this course, Passive Solar Energy: The Homeowners Guide to Natural Heating and Cooling, by Bruce Anderson and Malcolm Wells. It is available for free download from the website:

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